This invention relates to dynamoelectric machine end turn support assemblies and methods for making them.
Despite substantial improvements, there is a continuing desire for better end winding bracing of machines such as large turbine generators to minimize excessive vibration, wear of coil insulation, and fatigue cracking of series and copper strand phase connections. Examples of prior arrangements are those presented in Cooper et al U.S. Pat. Nos. 3,949,256 and 3,949,257, issued Apr. 6, 1976, which are incorporated by reference herein.
What is sought to be improved is to avoid loosening of strain blocks between top and bottom coil layers by radially clamping the top layer to the bottom layer in a relatively permanent tight assembly. The clamping should maintain pressure contact between coils and strain blocks and eliminate vibration of individual coils relative to the total coil end basket.
The present invention provides radial clamps located in the mid involute region of the end turns, that is, approximately midway between the core and the coil ends, to provide basket integrity in this region of a quality as has been previously provided by glass fiber reinforced rings at the coil extremities. It is particularly desired to provide such radial clamps by a method of assembly that ensures against their becoming loose and one that can be readily practiced in the field as well as in original manufacture.
The method for forming a radial clamp assembly in accordance with this invention includes placing a support block on two adjacent top coil end turns and another support block on two adjacent bottom coil end turns with each of the support blocks having an aperture with the apertures on a line passing between the two top coils and the two bottom coils. Then a loop of banding material is passed through the apertures in the support blocks and through the space between coils. The banding loop has some excess length extending beyond each of the support blocks. A pin is inserted within each end of the banding loop. At least one of the pins is turned to twist the banding loop so as to take up slack in the loop with the pins then bearing against the adjacent support blocks in a semi-tight assembly. Resin with which the banding is impregnated is then cured, such as by flow of hot air.
After the banding is so positioned and cured, a device, such as a hydraulic jack, is used to pull at least one of the pins radially away from the coils. This stretches the loop of banding material and puts additional tension on it. Then shims are placed between that pin and the adjacent block so the banding remains in tension upon release of the pulling device.
It is further desirable to place a layer of resin impregnated conformable material between each of the top and bottom support blocks and the coils adjacent thereto when the blocks are put in place. The resin in the layers of conformable material is cured along with the resin of the banding material.
Another aspect is to have springs, such as spring washers or spring plates, between one of the pins and the adjacent block and to use the pulling device and shimming on the other one of the pins.
The method described above results in an improved radial clamping arrangement for stator coil end turns, particularly for the mid-involute region of the end turns, comprising two support blocks between which a pair of top coils and a pair of bottom coils are clamped. The support blocks have a loop of banding material, that is twisted, cured and pretensioned, running through central apertures in them and in the space between the adjacent coils. Each end of the loop is over a transverse pin that bears against one of the support blocks. The clamping arrangement gives relatively permanent tightness to the coils and is advantageous in being readily applied in the field as well as in original manufacture to prevent vibration of the turns.